Design and Development of a New Rib for the Royal National Lifeboat Insitution

DESIGN AND DEVELOPMENT OF A NEW RIB FOR THE ROYAL NATIONAL LIFEBOAT INSITUTION

R Cantrill, D Hinge, J Barnes, R Cripps and H Fogarty, Royal National Lifeboat Institution, UK

SUMMARY

The Atlantic 21 and more recently the Atlantic 75 inshore lifeboat have been in service with the RNLI for many years. These are RIBs required to undertake a variety of roles in a wide range of operating conditions. As part of a larger review of the boat classes employed by the RNLI an operational requirement for a replacement to the existing Atlantic21/Atlantic75 was produced and a project initiated to develop a new boat to meet these requirements.

As a result a new 8.3 metre RIB, the Atlantic 85, has been developed capable of speeds in excess of 35 knots.

The paper outlines the early investigations, the design and development process, the structural development and the evaluation trials together with the engine selection and evaluation.

1. SYMBOLS & ABBREVIATIONS The initial Operational Requirement for FIB 1 is given in Figure 1. a Distance of each suspension strop from the c of g(m) ALB All-weather Lifeboat 3. FEASIBILITY STUDY cofg Centre of gravity FIB1 Fast Inshore Boat 1 3.1 BACKGROUND g Acceleration due to gravity (9.81 m/s2) GZ Transverse righting lever (m) It was considered that initially a feasibility study should ILB Inshore Lifeboat be undertaken into the design of an inshore lifeboat k Radius of gyration (m) designated Fast Inshore Lifeboat 1 (FIB1). kgf Kilograms force kn Knots The objectives of the feasibility study were: kPa KiloPascals 1 Length of suspension (m) • To identify constraints and key aspects of the design. LCG Longitudinal Centre of Gravity (m) M Mass (kg) • To identify the design options. m Metres 2 psi Pounds/inch • To produce a design requirement specification. s Seconds t Time (s) • To identify essential equipment to be fitted. tf Tonnes force • To identify implications for shoreworks and launching equipment. 2. INTRODUCTION • To confirm that the Operational Requirements were 2.1 PROJECT INITIATION achievable.

The Fast Inshore Boat 1 (FIB1) project was initiated in • To identify any changes to the Operational 1998 as part of a wider review of existing and future Requirements. lifeboat requirements for the RNLI. • To recommend a way ahead. 2.2 OPERATIONAL REQUIREMENT 3.2 INITIAL STAGE OF FEASIBILITY STUDY The primary operational requirements were for a twin- engine inshore lifeboat capable of speeds in excess of 35 The initial part of the feasibility study was to investigate knots in conditions commensurate with Beaufort Force 2 whether: and capable of operating in the maximum conditions commensurate with Beaufort Force 7. Manned by 3-4 • A boat existed on the market that could satisfy the crew, the boat was to be capable of being righted Operational Requirements without modification. following capsize and the engine re-started. A boat existed on the market that could be modified to satisfy the Operational Requirements. The Project Team considered that modifications to the Atlantic 75 to permit it to satisfy the Operational It was feasible to modify the existing Atlantic 75 to Requirements were not a realistic proposition because; meet the Operational Requirements. • The current design was too heavy for the hull shape A new design (either in-house or external) was required. • The required speed increase would require a further increase in weight.

3.2 (a) Literature Search and Market Survey • The scope for modification was limited.

A literature search was undertaken and a market survey • The extent of modifications required would mean was made of firms who offered any type of planing boat that, in essence, it would be a new design and in the defined length range. No stipulations other than therefore it would be better to start from a clean length were made at this stage. sheet.

The market survey was deliberately kept 'open minded', • The ballast arrangement would need to be reviewed with team members keen not to preclude any boat types with the probability that it would need to be from the survey. increased due to the CG moving aft.

Based on the survey responses, boats of interest were • The structure would need to be revised to cope with identified and categorised as RIBS, Semi-RIBS, the increased loadings due to greater weight and Catamarans and Rigid Monohulls. (A Semi-RIB being higher speed. defined as having a resilient collar that does not depend on inflation to maintain its shape) 3.3 ASSESSMENT OF DESIGN REQUIREMENTS To reduce the list of boats to a manageable size the project team reviewed the literature on each boat and Monohull or Catamaran produced a short list of candidate boats for further investigation. A monohull was considered to be preferable because of the difficulty of righting a capsized catamaran and doubts Visits were undertaken to view and trial short-listed about catamaran handling and seakeeping in extreme boats. conditions.

Of particular interest to the FIB1 project team were the RIB or Semi-RIB or Rigid Hull Swedish Rescue Service (SSRS) 8m Semi-RIB , the Dutch Rescue Service (KNRM) Antje Class and the A RIB or semi-RIB was considered to be preferred over a range of Naiad boats. Rigid Hull.

The objectives of the visits were to: Sheltered or Open Steering Position

• Learn as much as possible about the operation of An open steering position was preferred in order that the boats of this type by other rescue organisations. crew maintain contact with the weather conditions and to facilitate escape in event of capsize. • Examine the design and construction methods. Outboard or Inboard Engines • Trial the boats and to identify any that might be suitable for RNLI use. It was considered that either would be acceptable.

3.2 (b) Results of Market Survey and Visits Water Jet or Stern Drive

None of the boats evaluated were suitable to meet the The experience of other rescue organisations and Operational Requirement. commercial operators with stern drives was not very good with the robustness and life being poor. However, information gathered during the visits was of help in producing the Design Requirement Specification. On these grounds, water jets were the preferred option with inboard engines. 3.2 (c) Assessment Of Modification To The Existing Atlantic Class

-2- Petrol or Diesel This took the form of 3 full days of structured trials in the Either fuel type was considered to be acceptable subject Solent with frequent review periods. to practical limitations on the storage of large quantities of petrol. The boats were compared in various sea conditions, from calm water off Cowes to extremely rough on the Shingle Hull Material - FRP or Aluminium Bank off the Needles.

The choice of construction material was between FRP The unanimous conclusion at the end of the trials was and Aluminium. that the Atlantic 75 was the better boat for this application. It was considered that FRP was the logical choice given that the number of boats to be built would justify the The Group recommended that FIB1 be a development of production of mould tooling. this design and also requested that provision was made to carry a fourth crew member. The benefits of FRP are stated to be; The User Group was an essential element in the • Wide expertise locally development of the design and was convened at key • Repeatability points in the Project to comment and advise. • Weight • Cosmetics 6. PROTOTYPE BOAT DESIGN, CONSTRUCTION AND TRIALS

4. DESIGN PROPOSALS 6.1 PROTOTYPE BOAT DESIGN

4.1 A total of 21 manufacturers were invited to submit 6.1 (a) Hull design proposals against the Design Requirement Specification. In order to retain the handling characteristics of the Atlantic 75 it was decided that the new hull would be a 4.2 The project team received 14 design proposals, which geosim of the existing one. were formally reviewed. After consideration of various deck layouts it was 4.3 As a result of the review it was decided that the concluded that the hull needed to be 13% bigger in order preferred material of construction was FRP because: to accommodate the additional crew and equipment.

• The number of boats to be built would justify To minimise the increase in height above the waterline the production of mould tooling the collar was scaled in length and width, but was • Wide expertise locally maintained at the same diameter as the Atlantic 75, in • Repeatability order to facilitate casualty recovery from the water. • Weight • Maintenance 6.1 (b) Structure • Cosmetics The prototype boat presented an opportunity to 4.4 It was also decided that the size of boat being investigate the feasibility of using thermoplastics for considered was too small to allow the proper installation lifeboat hull construction. of twin inboard engines/jet units with adequate accessibility for maintenance. The propulsion choice was The potential of this tough material to reduce therefore to be outboard motors. maintenance costs was attractive.

4.5 The best three proposals were based on current The prototype hull was built using glass fibre reinforced commercially available designs and the decision was polypropylene ('Twintex'). taken to obtain an example of each boat for evaluation against the Atlantic 21 and 75. The hull shell was single skin and the structure was Twintex/balsa sandwich. 5.0 USER GROUP 6.1 (c) Console A 'User Group' of 12 experienced Atlantic helmsmen was selected to undertake the evaluation of the three test The User Group was consulted regarding the seating boats. configuration for the console. The various options were

-3- presented as full size mock-ups for evaluation. See • The bag must continue to be effective if the boat Figure 2. capsizes a second time. This means that the re- seat setting of the pressure relief valve(s) must As a result the prototype was based on an Atlantic be high enough to cope with the hydrostatic console with a fourth seat grafted onto the back. (and some dynamic) pressure. The relief valve is selected to re-seat at 20.6 kPa (3.0 psi) and 6.1 (d) Engine Selection opens at 29.6 kPa (4.3 psi).

The predicted power requirement was for 2 x 115 hp The choice of inflation gas was between CO2 and dry air. outboards There are various advantages and disadvantages to each:

The trials programme included an evaluation of both 2- • CO2 will kill in confined spaces but this is not a stroke and 4-stroke engines, but initially the prototype problem in an open boat. Accidental discharge boat was fitted with a pair of 120 hp 2-stroke engines, of this volume in a boathouse would also not which became available at a reasonable price cause dangerous concentrations. • CO2 can freeze in hoses and valves. It often has 6.1 (e) Righting After Capsize a small amount of nitrogen added to reduce freezing problems, especially when released at The righting system design is closely linked to the crew low ambient temperatures. Valves and inlet procedure used in the event of capsize as proven in nozzles are sized to control the flow of gas to service on the Atlantic 75. prevent freezing up. • Dry air can potentially fill a bag faster. Crew lifejackets are designed with sufficient permanent • Jets of freezing gas and ice can cut through the buoyancy to float but not to self-inflate, unless a toggle is righting bag. Special inlet nozzles are fitted and pulled. This enables the crew to escape from under the diffuser pads fitted around them. upturned boat. There is breathing space under the • CO2 is more compressible than air and needs capsized boat above the inverted waterline. about half the size of storage cylinder for the same final volume and pressure. Because the boat tends to right very quickly when the bag is inflated, the crew gather clear of the boat at the Based on the smaller bottle size and the previous transom, attached by a safety line, before firing the successful use of CO2, this gas was also chosen for the righting system. Once the boat has righted, they climb Atlantic 85. back on board. The righting system uses one bag but has two separate The system is designed to achieve: gas inflation systems, either of which can fully inflate the 20 kPa (2.9 psi) bag pressure at -10°C with a bottle fill bag. of 4.20 kg CO2 + 0.17kg N2.

The bag design is based on the following requirements: Figure 5 shows results of a bag inflation trial. The • The collar at the aft end of the capsized boat pressure relief valve was blocked in order to properly must lift nearly clear of the water. At this point measure the potential maximum pressure and incidentally the boat becomes unstable and starts to right, to test the bag strength. Pressure measurements were pivoting about the bow. The bag must be large recorded in psi. enough to lift the aft hull, engines (200 kg each), roll bar etc. The required bag volume was 3 The design righting pressure is shown at 2.9 psi (20.0 selected at 1.53m . kPa). The required pressure of 4.7 psi (32.4 kPa) was the • The GZ curve, with bag inflated, must be calculated pressure at 17°C, needed to achieve 2.9 psi (20 positive over the whole range. This means that kPa)at-10°C. the bag must have sufficient width to prevent the boat achieving a stable position its side. Tests Bl, B2 & B3: Full charge of gas but with various Figure 3 shows the GZ curve as the boat cylinder valves, release heads and inlet valves. capsizes with 4 crew (bag deflated). Figure 4 shows the same fuel and ballast load but with no Test B1: Never reached full pressure due to a leak in the crew (who are now swimming) and the bag inspection hatch, which started to release gas after about inflated. The fuel tanks and water ballast tank 30 seconds. are low in the boat. This means that the GZ curves are very similar with full and light liquid Test B2: Pressure would have climbed higher but was loads. released at 8 psi (55.2 kPa) for safety reasons. • The bag must deploy in a reliable way (not wrap around the roll bar) and then achieve a stable shape.

-4- Test B4: Used an Atlantic 75 bottle (3.9 kg CO2), which would right the boat, but too slowly, especially in cold Figure 6 shows trim and speed plotted for a range of conditions. LCG positions. LCG is measured from the transom at keel. As can be seen, the increase in the higher speeds From the graph, a righting time in excess of 17 seconds with an aft shift in LCG is not very great (although might be expected (curves Bl & B2) bearing in mind that different propellers could have shown a greater the boat comes up quickly once righting pressure is differential). Conversely, the improvement in sea- reached. The actual time will depend mainly on the keeping with the forward LCG was marked. ambient air and water temperatures. 6.2 (d) Righting Trial 6.2 PROTOTYPE TRIALS The boat was fitted with a ballast box (406 kg), bolted to 6.2 (a) Boat set up the transom, to simulate the weight and centre of gravity of the engines. The inversion proofed engines were not The boat was normally tested in the most demanding fully developed at the time of the trial. condition, which for speed trials was full load. Ballast was carried to compensate for missing crew members The boat was rolled over using strops onto a crane. The and equipment fit. point of capsize was about 90 degrees (a higher angle than with the crew on board). 6.2 (b) Propulsion Trials The righting system was fired and the boat righted in 22 Prototype propulsion trials were carried out with two 120 seconds. See Figure 7. HP 2-stroke outboard engines. It immediately became clear that achieving in excess of 35 knots was not a The only problem arose due to the roll bar, which had problem. However, acceleration was disappointing and lost some cross bracing due to concerns over crew access hard turns caused excessive cavitation and ventilation of underneath. This resulted in a distorted structure. Cross the propellers. bracing has since been reintroduced.

6.2 (c)LCG Trials 7. PRE-PRODUCTION BOAT CONSTRUCTION The performance of the boat in waves is vital to crews, AND TRIALS who sometimes have to work in breaking water and surf. The LCG of the vessel is important in this respect, as is 7.1 PRE-PRODUCTION BOAT CONSTRUCTION the radius of gyration (see below). 7.1 (a) Hull In general terms, moving the LCG forward trims the bow down with the following effects: The performance of the prototype boat confirmed the • Better sea-keeping (vessel less inclined to fly acceptability of the hull form at the larger scale. off waves, wind less likely to hold the bow up). • Faster on to plane. The User Group had moved very strongly in favour of a • Some loss in top speed. variable water ballast system and this was incorporated in • If taken too far forward: A tendency to bow the pre-production boats. steer & broach. 7.1 (b) Boat Structure However, there are other major influences on trim on the vessel: The Twintex material showed great potential for lifeboat • Engine trim (trimming the engines in lowers the use but further work was required to develop the bow). moulding process to the point where boat yards could • Bow ballast tanks: The forward ballast tank can undertake it. be filled by a retractable transom scoop, controlled by the helmsman, to add about 200kg The Project timescales precluded doing this development of seawater forward. with FIB1, but it is continuing under other auspices and is a very exciting prospect for the future. To control these variables, the boat was normally tested with the engines at 0° trim and no water ballast. Farther In parallel with the prototype boat development, SP testing was then carried out to check for any vices with Systems were developing a manufacturing method based the ballast tanks full and engines trimmed in (maximum on their SPRINT materials. bow-down boat trim). The Atlantic 85 has a very sea- kindly hull and is very difficult to broach, even in this The aim was to reduce costs and improve quality by extreme condition and in breaking waves. achieving tight production tolerances on dimensions.

-5- The chosen engines had to have the acceptance of the This was achieved by developing a kit of pre-cut mat and RNLI coast staff and crews as well as meeting foam cores. A comprehensive handbook was also operational requirements. produced, detailing the lay up and curing process. The engines had to be capable of being inversion proofed The internal sandwich structure is moulded using the (by no means certain for 4-strokes). same method and is glued into place in the hull. It was an opportunity to foster relationships with new The 3 pre-production boats were built using this system, suppliers. the final arrangement being an all carbon/foam structure. In addition to the initial 120hp 2 stroke engines, 115 hp 7.1 (c) Console 4-stroke and 115 hp 2-strokes were also tested.

Once the prototype had confirmed the seating The final choice was 115 HP 4-stroke engines. arrangement, the console layout was primarily concerned with accommodating the radar installation. 7.2 PRE-PRODUCTION BOAT TRIALS The original concept had been for the crew to perform the navigation tasks using the radar and chart plotter and For these trials, the benchmark for the boat was the to display the results to the helmsman on a repeater existing Atlantic 75. screen on the helm console. 7.2 (a) Acceleration Trials However it became clear that the helmsman needed to concentrate on helming the boat and not be distracted by Several different engine and propeller combinations were too much information. tested. The top speed was never a problem but careful propeller selection was needed to achieve sufficiently The final arrangement has heading and depth displayed good acceleration. In the end this was achieved without to the helmsman and the crew operate the radar/plotter. sacrificing much top end speed.

7.1 (d) Outfit and equipment Different engine heights were also tried. Top speeds were reduced by about a knot for a 30 mm drop in engine • Intercom fitted in helmets with VHP talk facility height. At the lowest height, ventilation and cavitation for the front 3 crew were much reduced in acceleration and tight turns. Handling in broken water was also much improved. The • VHP final height of ventilation plate below buttock line is 26 mm. • Handheld VHP Figure 8 shows typical plots from the acceleration trials, • VHP DP in this case demonstrating the extra torque from a 4 stroke over a 2 stroke engine. These trials were run • Radar/GPS Plotter between transit marks off West Cowes.

• Portable salvage pump 7.2 (b) Bollard Pull & Single Engine Trials

• First Aid Satchel with Oxygen Towing is often undertaken and so bollard pull has some importance in it's own right, as well as being related to low speed performance. Figure 9 shows a bollard pull • Searchlights trial comparing 2 stoke, 4 stroke and Atlantic 75 (70 HP) 2 stroke engines. • Anchor stowage in bow locker The Atlantic 85 operational requirement calls for the • Spare propeller ability to plane on one engine. There are various definitions of what speed constitutes "planing" but the 7.1 (e) Engine Selection boat achieves 21 knots under one engine, which is safely over the main resistance hump. Both 2-stroke and 4-stroke petrol engines were considered. Diesel outboard motors could not be used as the specific power of the latest models is still too low for this application.

-6- 7.2 (c) Radius of Gyration Trials Being economical, the 4 stroke engines also save the Experience shows that better handling characteristics can necessity to increase fuel storage quantities at stations. be achieved with the correct radius of gyration. However, This is an important consideration because many RNLI this is not always easy to achieve: boathouse storage bins are already as large as is permissible under current regulations. • The ideal radius will vary for different helmsmen. Some like a steadier vessel, others a Typical fuel consumption measurements are plotted in more responsive one. Figures 11 and 12. • The radius can be hard to move, as most equipment is positioned for other more pressing Performance Summary reasons. However, ballast was an option on this boat, which allowed the possibility of some Performance Summary: Atlantic 75 and 85 Boats tuning. Item A75 A85 Maximum speed (kn) 30 approx. 35+ Consideration was given to measuring the pitch radius of Acceleration to 152m(s) 13.7 11.7 gyration, which might give a better indication of vessel Planing speed + 1 engine (kn) 20 21 response to waves. This can be done with the Lamboley Bollard pull (tf) 0.71 0.88 pitch test but would have needed a more complicated test rig and time was too short. For these boats the pitch radius would not have been very different to the yaw 8. PRODUCTION BOAT radius.

Two trials were carried out on the boat keeping the LCG 8.1 PRODUCTION BOAT CONFIGURATION and displacement constant. In each case two ballast masses were used of 50kg each. These masses were The final specification of the production boats is given in firstly concentrated amidships and then split fore and aft. Figure 13 and illustrated in Figure 14.

• Sea trial: Subjective evaluation of the two ballast conditions in rough water by experienced 9. CONCLUSIONS helms. • Workshop trial: Measurement of the two The FIB1 project has successfully delivered an inshore different radii of gyration. See Figure 10. lifeboat that meets the operational requirement and largely exceeds the previously un-collated expectations The sea trials produced a clear preference for the lower of its volunteer crew. radius of gyration. When manoeuvring in short breaking waves, the gave the boat a better turning ability enabling In achieving this, advanced sandwich composite quick response to avoid the worst of the wave crest. materials have been introduced to the inshore lifeboat fleet, improving performance and reducing predicted The swing trial gave swing periods of 8.09 and 8.35 through life maintenance costs. seconds (mean often oscillations), which calculated to radii of 2.208 m and 2.280 m respectively. This 72 mm difference could be clearly felt by the helmsmen, even 10. ACKNOWLEDGEMENTS though it equals only 1.0% of rigid hull length. The work, assistance and enthusiasm of the following Interestingly, the radius of gyration of many of the larger people and groups are acknowledged: - RNLI All-weather Lifeboats (ALBs) been deliberately increased in the past to improve rough water directional • RNLI Divisional Staff, Inshore Lifeboat Centre stability. and Atlantic 21/75 lifeboat crews • FIB 1 Project Team, RNLI The calculation method used to find the radii of gyration • SP Systems, Isle of Wight is shown in Appendix A.

7.2 (d) Fuel Consumption Trial 11. REFERENCES The ability to carry the larger 115 HP 4-stroke engines means that the Atlantic 85 uses the same amount of fuel 1. Project Study Report for Fast Inshore Boat 1 (FIB 1), per nautical mile at full throttle as the previous Atlantic RNLI, May 2000. 75 with 2-stroke engines. This is on a boat which is 400 kg heavier and which is travelling about 5 knots faster.

-7- 12. BIOGRAPHIES

Rob Cantrill Principal Engineer at the RNLI and FIB 1 Project Manager. Doug Hinge Senior Engineer at the RNLI. Has worked extensively with small fast craft, running a small firm of naval architects before joining the RNLI in 2003. John Barnes Assistant Production Manager at the RNLI Inshore Lifeboat Centre, East Cowes, and Project Trials Manager. Prof. Bob Cripps Engineering Manager, RNLI Hugh Fogarty Staff Officer Operations (Fleet) at the RNLI. Has Operational responsibility for the Lifeboat fleet.

-8- OPERATIONAL REQUIREMENT FOR THE FAST INSHORE LIFEBOAT. FIB1 1. To be a suitably equipped inshore lifeboat with an external manually operated self-righting mechanism. 2. To be capable of being launched from a beach and a slipway using a trolley/carriage as required and from a davit. 3. To have two or more engines. 4. To be capable of being beached in an emergency without damage to the propulsion system or steering gear. 5. To be capable of safe operation in daylight in sea conditions associated with a wind speed of Beaufort Force 6/7 and at night in sea conditions associated with a wind speed of Beaufort Force 5/6. 6. To be able to maintain a maximum speed of at least 35 knots, with the boat in full load displacement condition, in the sea conditions associated with a wind speed of Beaufort Force 2. 7. To be capable of getting on the plane and maintaining planing speeds under single engine operation with the boat in full load displacement condition, in the sea conditions associated with a wind speed of Beaufort Force 2. 8. To have a duration of not less than 2.5 hours at full power in all weathers with 10% useable fuel remaining at the end of this period. 9. To have good towing capability. 10. To have seating for three* crew members. 11. To have sufficient space available for carrying 4 survivors and one prone casualty safely. 12. To have good access to waterline for casualty handling.

* Later increased to four, subject to no other requirements being compromised.

Figure 1: Operational Requirement for FIB1

Figure 2 - Console Mock-up

-9- Heel to Starboard

Figure 3 - GZ Curve, 4 crew, 95% fuel, 95% ballast, 2.30t, LCG 2.46m, VCG 0.78m, Righting bag deflated

Heel to Starboard

Figure 4 - GZ Curve, 0 crew, 95% fuel, 95% ballast, 1.94t, LCG 2.41m, VCG 0.64m, Righting bag inflated

-10- FIB1 Righting Bag Pressures

0.0 0 5 10 15 20 25 30 40 50 60 70 80 90 100 110 120 180 240 Max Time (seconds)

Figure 5 - Gas Bag Pressures: Test Firing

FJB1-01 - LCG Trials - 2.32 tonnes

-O- LCG 2.05 Speed LCG 2.05 Trim LCG 2.12 Speed LCG 2.12 Trim -D- LCG 2.34 Speed -Q- LCG 2.34 Trim